How much to charge for advertizing space on my website?

I have no idea....

[carpediem]

Hi all,

Recently I had someone contact me through website contact form and asked how much I would charge for putting their banner on my website.

I have no experience with this... My site is in weight loss niche:

21,000 UVs/month
61,000 pageviews/month

Should I go for a fixed price, for example: "For 1 month the price is such and such..."

or should I go with eCPM value? Maybe put Adsense on my site for a week and see what eCPM will be, then charge little more than that?

Thanks in advance.

[wyweb]

I go for whatever they can stand. When they say "ouch" I back off.